A nitride semiconductor light emitting element that emits light in a wavelength region of visible to ultraviolet light holds potential of application in a wide range of fields, such as in health, medicine, industry, illumination, precision machinery and the like, because of the advantageous in terms of its low power consumption and small size. A Nitride semiconductor light emitting element for partial wavelength regions, for instance blue light wavelength region, is already in commercial use.
However, as to the nitride semiconductor light emitting element, not limited to the nitride semiconductor light emitting element that emits blue light (hereafter, referred to as “blue light-emitting diode”), it is desired for enhanced emission efficiency and light output. In particular, at present, the practical use of a nitride semiconductor light emitting element that emits light in ultraviolet wavelength region (hereafter, referred to as “ultraviolet light-emitting diode”) is hampered by the problem of its considerably poorer external quantum efficiency and light output as compared with the blue light-emitting diode. The low efficiency of light-emitting layer (hereafter, referred to as “internal quantum efficiency”) is one of the causes underlying the significantly poor external quantum efficiency and light output.
The internal quantum efficiency of the light emitting layer formed of a nitride semiconductor crystal is influenced by threading dislocations. In a case of high dislocation density of the threading dislocations, non-radiative recombination is dominant, and therefore a drop in the internal quantum efficiency is caused.
In a case where a substrate made of a material such as sapphire or the like causing a significant lattice mismatch with a nitride semiconductor is used as a substrate for epitaxial growth, the aforementioned threading dislocations are likely to occur in particular at a growth interface. Therefore, in order to obtain a nitride semiconductor crystal layer having a low threading dislocation density, it is extremely important to control the behavior of each of the constituent elements in the early stages of growth. In particular, in growth of a nitride semiconductor crystal layer containing Al (particularly AlN layer), a diffusion length of the constituent element composed of a group III atom is shorter than that of a nitride semiconductor crystal layer that does not contain Al (particularly GaN layer). Therefore, a plurality of nuclei are generated in a relatively high density at the early stage of growth. Then, it has been known that most of threading dislocations are likely to occur on an interface between adjacent nuclei when the adjacent nuclei are combined. Moreover, with using MOCVD equipment (Metal Organic Chemical Vapor Deposition equipment) as a manufacturing apparatus, trimethylaluminum (TMAl) gas and an ammonia (NH3) gas, which are respectively a typical group III material and a typical group V material, react with each other undesirably in a gas phase to give particles (nanoparticles) having a size in a nanometer order. The nanoparticles present on a surface of the substrate may inhibit growth in an AlN crystal. Therefore, an ultraviolet light-emitting diode that includes Al as the constituent element in the nitride semiconductor crystal layer has more threading dislocations present in the nitride semiconductor crystal layer than a blue light-emitting diode that does not include Al as the constituent element. Therefore, the ultraviolet light-emitting diode has lower emission efficiency than the blue light-emitting diode.
For the aforementioned problem, a method for producing a semiconductor structure including a step of forming an AlN buffer layer capable of emitting light having an emission wavelength in a range of 230 to 350 nm in an ultraviolet region and suitable for a light-emitting device structure (Patent Document 1) was proposed. An AlN high-quality buffer growth structure described in Patent Document 1 includes a sapphire (0001) substrate as well as an AlN nucleation layer, a pulsed supplied AlN layer, and a continuous growth AlN layer which are successively formed on the sapphire substrate.
The AlN nucleation layer, the pulsed supplied AlN layer, and the continuous growth AlN layer are formed with MOCVD equipment. The AlN nucleation layer is grown in an initial nucleation mode, which is a first growth mode, by means of an NH3 pulsed supply method. The pulsed supplied AlN layer is formed by using NH3 pulsed supply in a slow growth mode, which is a second mode. The continuous growth AlN layer is grown in a fast vertical growth mode. Patent Literature 1 discloses that the second mode is a mode for increasing a grain size and reducing dislocations, and can make the uneven AlN nucleation layer flat. Moreover, Patent Literature 1 discloses that the fast vertical growth mode is a mode for more improving the flatness and suppressing cracks and do not use the AlN growth method by means of the NH3 pulsed supply. The AlN growth method by means of the NH3 pulsed supply method is a method in which a TMAl gas, which is an Al source, is continuously supplied while a NH3 gas, which is an N source, is supplied in a pulsed manner.
For example, Japanese patent application publication No. 2009-54780 (Patent Document 1) discloses growth temperatures of the AlN nucleation layer, the pulsed supplied AlN layer, and the continuous growth AlN layer are selected to 1300° C., 1200° C., and 1200° C., respectively.
Patent Document 1 also discloses that a deep ultraviolet LED with an emission wavelength of 250 nm includes an AlN buffer layer formed on a sapphire substrate and a LED structure formed on the AlN buffer layer. The LED structure includes a MQW (multiple-quantum well) of a Si doped n-type Al0.75Ga0.25N layer and an Al0.75Ga0.25N/Al0.60Ga0.40N-layer, an electron blocking layer made of a Mg doped Al0.95Ga0.05N, a Mg doped Al0.75Ga0.25N-layer, and a Mg doped p-type GaN layer which are arranged in this order starting from the AlN buffer layer side. Furthermore, a first electrode is formed on the Mg doped p-type GaN layer, and a second electrode is formed on the Si doped n-type Al0.75Ga0.25N layer.